Solid propellant grain with cellulose acetate coating



Nov. 17, 1964 w, PROELL 3,157,127

SOLID PROPELLANT GRAIN WITH CELLULOSE ACETATE COATING Filed Marfch 15,1962 INVENTOR. Wayne A. Proe/l United States Patent 3,157,127 OlLlDhRGPELLANT GRAZN WETH CELLULGSE ACETATE CUATKNG Wayne A. Email, eymour,had, assignor to Standard @il Company, Chicago, llh, a corporation oflndiana Filed Mar. 15, 1962, fies. No. 13%4923 6 Claims. (Cl. 192-98)This invention relates to ammonium nitrate articles, and particularlyammonium nitrate propellant grains suitable for use in gas generatorsand rockets.

The explosive industry utilizes ammonium nitrate as a principalcomponent of explosives. The ammonium nitrate may be used in conjunctionwith oxidizable organic materials, or ammonium nitrate particles may becombined with other explosives. It is advantageous frequently to isolatethe ammonium nitrate from direct contact with other components of thefinal explosive, or simply to protect the ammonium nitrate from contactwith at mospheric moisture. At present, it is common to apply a sealercoat of hydrocarbon wax to the ammonium nitrate particles. This is not asatisfactory answer because the particles tend to cohere in storage.

Ammonium nitrate propellant compositions are now widely used for thegeneration of gas at high pressure, which gas is used to drive turbinesand the like, or to propel rockets. These propellant compositionscomprise ammonium nitrate, an oxidizable organic binder which permitsforming the propellant to configurations of definite shape, and anammonium nitrate combustion catalyst. it is necessary to control thedirection of burning of the propellant body; this is done by applying tothe surface(s) of the grain which are not desired to burn, a coat of acombustion restrictor. The restrictor coat must be non-porous and freeof holes of any size because hot gases in the motor chamber will passthrough these holes and ignite the propellanP-the burning of thepropellant at the holes causes uneven pressure maintenance in the motor.Also, the restrictor coat must be stable at prolonged storage and notdevelop cracks. Some restrictor materials fail on storage owing toreaction with exposed ammonium nitrate in the propellant body or evenwith a binder component. In such a case it is customary to apply asealer coat of some material-usually a restrictor in its own right-tothe surface of the grain. Then the restrictor coat is applied to thesealer coat.

Cellulose acetate may be adhered to ammonium nitrate and functions, incoats of suitable thickness, as a restrictor. It is difiicult to buildup a coating of suiiicient thickness by the use of solutions ofcellulose acetate which are applied to the surface to be restricted(coated). Commonly sheets of cellulose acetate are used as restrictorsand are cemented to the grain surface by the use of a cellulose acetatesolution cement. Such cementing techniques are barely satisfactory andfrequently result in restrictor failure during the burning of thepropellant.

When cellulose acetate is applied from solution, it is frequentlydifiicult to obtain a non-porous barrier. The coat is frequently brittleand cracks during handling of the coated grain before the application ofthe restrictor coat. In any case, considerable skill, care and expensegoes into the production of a satisfactory adherent coating of celluloseacetate when using a solution, such as cellulose acetate and acetone, asthe source of the cellulose acetate coat.

An object of the invention is an ammonium nitrate particle having acellulose acetate sealer coat adhered to a portion of, or even theentire surface thereof. A particular object is an ammonium nitratepropellant having a sealer coat on at least a portion of the surfacethereof, which sealer coat is formed of cellulose acetate. Stillanilhilli? Patented Nov. 1?, 196 4:

other particular object is a shaped ammonium nitrate propellant grainprovided with a cellulose acetate sealer coat and a restrictor coat ofmaterial tending to react with ammonium nitrate, said sealer coatbarring contact between exposed ammonium nitrate and the restrictorcoat. Other objects of the invention will become apparent from thedetailed description thereof.

It has been discovered that ammonium nitrate itself or the surface ofammonium nitrate type propellant can be provided with a satisfactorysealer coat of cellulose acetate by the following procedure. First, alayer of a liquid capable of plasticizing cellulose acetate is appliedto the surface to be covered. Second, powdered cellulose acetate isapplied to the liquid covered surface; sufiicient of the acetate isapplied to form a seal coat on the surface after the baking treatment.Lastly, the cellulose acetate covered surface is baked at a temperatureof about C. for a time sutiicient to form a coat of cellulose acetate onsaid surface.

The sealer coat present on the article or propellant of the invention isformed from cellulose acetate. Particularly suitable for this purposeare the cellulos acetates which have a combined acetic acid content ofabout 50-53 weight percent. A preferred cellulose acetate with acombined acetic acid content of about 55% is made by The CelaneseCompany as grade HLFS 93. Another preferred cellulose acetate is thatsold to the trade as lacquer grade containing 54-56% combined aceticacid.

The cellulose acetate is applied to the wet surface in the form of apowder and the degree of fineness of the powder is determined by thetype of acetate used, the type of plasticizer, the thickness of thesealer coat desired, and the baking temperature and time. Thicker coatsmay be made up of two or more layers, formed by repeated applications ofplasticizer and cellulose acetate.

The ultimate thickness of the sealer coat will be that needed to meetthe requirements of the particular task. A paper-thin coat may bsuificien't to act as a barrier between exposed ammonium nitrate and arestrictor coat which is capable of reacting with ammonium nitrate. U.S.Patent No. 3,012,507 of Robert A. Masher and William G. Stanleydescribes in detail the use of a sealer coat (undercoat) as a barrierbetween ammonium nitrate and restrictor coat material reactivetherewith. When it is desired to produce an ammonium nitrate articlecovered with a sealer coat to act as a barrier against absorption ofatmospheric humidity, the coat will be tailored to suit the humidityconditions faced by the ammonium nitrate article in storage. In otherinstances, the sealer coat will function as a restrictor coat also. lnsuch a case, the thickness is great enough to withstand the hot gasesfrom ammonium nitrate decomposition for the time needed to protect thesurface beneath the sealer coat segment. When used as a chemical barriermaterial, the cellulose acetate sealer coat may be in general or less.When used as a restrictor coat, in general, the thickness will be on theorder of although in severe cases, thicker coats may be necessary.

A liquid capable of plasticizing cellulose acetate is ape plied to thesurface to be covered by the sealer coat as the first step in theformation of the sealer coat. It may be necessary to heat theplasticizer in order to apply it to the surface to be covered. Ingeneral, the temperature at which the plasticizer is applied,preferably, is lower than the lower baking temperatures. The amount ofplasticizer applied is dependent upon the particular plasticizer andalso upon the thickness of the final sealer coat desired. in general, alayer of liquid exhibiting complete wetting of the surface, to visualobservation, is sufficient for the formation of a sealer coat suitablefor use as a-chemical barrier. The liquid pla ticizer may be applied tothe surface by brushing, spraying or dipping.

Any compound capable of plasticizing cellulose acetate may be used.Plasticizers may be classified generally as polymeric esters, esters ofpolyhydric alcohols, ethers of nitrophenols, nitromonocyclic aromatics,esters of polycarboxylic acids, alkyl ethers of polyglycol, andpolyglycols. Specific examples of such plasticizers are ethylene glycoldiglycolate, acetin (mono, di and tri), nitromethylpropanedioldiacetate, triethylene glycol di-Z-ethylbutyrate, triethylene glycoldi-Z-ethylhexoate and polyethylene glycol di-Z-ethylhexoate. Specificexamples of esters of polycarboxylic acids are alkyl citrates such astriethyl citrate, acetyl triethyl citrate, trimethyl citrate,

' acetyi trimethyl citrate, and the dialkyl phthalates such as dibutylphthalate and dioctyl phthalate. Specific examples of nitrophenyl etherswhich may be incorporated in the binder of the body portion aredinitrophenyl allyl ethers, dinitrophenoxyethanols and thedinitrodiphenyl ethers such as bis(dinitrophenoxy)ethane and 2,4dinitrodiphenyl ethers. Such nitromonocyclic aromatics asdinitrotoluenes and dinitrobenzenes may be used. Combinations of two ormore of the plasticizers may be used.

In those cases when the viscosity of the plasticizing liquid is too lowto provide a sufficiently thick layer of liquid on the surface to becovered, the viscosity of the liquid may be increased by dissolving somecellulose acetate therein. It is to be understood that even considerablebodying of the plasticizer with cellulose acetate will not permitformation of satisfactory sealer coats for most applications. Coatsprepared from plasticizer solution only tend to be porous.

The acetate-plasticizer surface covering is converted to a hard, tough,pore-free, adherent coating by heat treatment (baking) at a temperatureof about 90-120 C; The heat treatment may be directed only at thecovered surface, or the entire article may be heated to the necessarytemperature. The heat treatment is continued for a time sufiicient toconvert the cellulose acetate particles to a continuous coating. Thepresence of the plasticizer liquid appears not only to provideadherency, but also assists in the melting of the solid particles andtheir fusion into a tough, continuous, nonporous sealer coat. The exacttime needed will be dependent upon the temperature, the particle sizeand the thickness of the acetate-liquid covering prior to heattreatment.

The sealer coat formed as described hereinabove is of particularusefulness in the case of solid propellants comprising a shaped bodyportion formed of ammonium nitrate and an oxidizable organic binderWhere the surface of said body is characterized by the presence ofexposed ammonium nitrate, i.e., ammonium nitrate which has not beencovered by the binder. And it is desired to utilize as the combustionrestrictor coat, a material which reacts with ammonium nitrate toproduce gaseous products or to produce areas of weakness which result inpuncturing of the restrictor coat or brittleness. U.S. Patent No.3,012,507 describes in some detail the restrictor materials which doface the problem of reaction with ammonium nitrate. And itis intendedthat the teachings of this patent and other patents on particular typesof restrictor coatings such as US. Patents Nos. 2,999,462

and 3,012,506 are incorporated herein. The sealer coat of the .inventionis also particularly suitable for use as an undercoat where the desiredrestrictor coating does not adhere well to the propellant surfacebecause of the incompatability of the restrictor material and the bindermaterial of the propellant.

'The'body portion of the solid propellant of the invention consistsessentially of ammonium nitrate as the major component and oxidizableorganic binder is also present to permit the ammonium nitrateto beformedinto shaped configurations or grains.

monium nitrate is preferably in a finely divided particulate form whichmay be either produced by prilling or by grinding. Usually thecomposition will contain between about 65 and of ammonium nitrate.

When ammonium nitrate decomposes free-oxygen is formed. Advantage of theexistence of this free-oxygen is taken and oxidizable organic materialsare used as the binders. These oxidizable organic materials may containonly carbon and hydrogen, for example, high molecular weighthydrocarbons such as asphalts or residuums, and rubbers either naturalor synthetic. Or, the oxidizable organic material may contain otherelements in addition to carbon and hydrogen for example, Thiokol rubberand neoprene. The stoichiometry of the composition is improved, withrespect to smoke production, by the use of oxygenated organic materialsas the binders. The binder may be a single compound such as a rubber orasphalt or it may be a mixttire of compounds. The mixtures areparticularly suitable when special characteristics are to be imparted tothe grain which cannot be obtained by the use of a single compound.

The multi-component binder commonly consists of a polymeric basematerial and a plasticizer therefor. Particularly suitably polymericbase materials are cellulose esters or alkanoic acids containing from 2to 4 carbon atoms such as cellulose acetate, cellulose acetate butyrateand cellulose propionate; the polyvinyl resins such as polyvinylchlorideand polyvinyl acetate are also good bases; styrene-acrylonitrile is anexample of a copolymer which forms a good base material.

A preferred binder component is broadly defined as an oxygenatedhydrocarbon. The hydrocarbon base may be aliphatic or aromatic or maycontain both forms. The oxygen may be present in the plasticizer inether linkage and/or hydroxyl group and/or carboxyl groups; also theoxygen may be present in inorganic substituents particularly nitrogroups. In general any plasticizer which is suitable for Work with thedefined polymers may be used in the invention. Exemplary classes ofplasticizers which are suitable are set out in US. Patent No. 2,999,462.

In addition to the basic components, i.e. ammonium nitrate binder andcatalyst, the gas generator propellant composition may contain othermaterials. For example, materials may be present to improve lowtemperature ignitability, for instance oximes may be present or, asphaltmay be present. Surfactants may be present in order to improve thecoating of the nitrate with the binder and to improve the shapecharacteristics of the composition. Various burning rate promoters,Which are not catalyst per se, may also be present.

The mixture of ammonium nitrate and binder is expiperidine chromate arealso excellent chromium cata-- lysts. Certain heavy metal cyanidesparticularly those of cobalt, copper, lead, nickel, silver and zinc areelfec tive catalysts. The cyanamides of barium, copper, lead mercury andsilver are efifective catalysts. The various Prussian blues areexcellent catalysts.

Organic catalysts are particularly useful when it is desired to havecombustion products which are gases or vapors and thereby do not erodegas eXit orifices. Two catalysts which do not contain any metalcomponents are pyrogene blue (color index 95 696l) and methylene blue.Particularly suitable catalysts are the alkali metal barbiturates.

A particular embodiment of the propellant utilizing the sealer coatdescribed herein is set forth for purposes of illustration. In thisembodiment, the body portion of the propellant which is a solid rod inshape consisted of GP. ammonium nitrate, 62%; sodium barbituratecatalyst, 3%; toluene diamine, 1%; carbon black, 4%; with the binderconsisting of: lacquer grade cellulose acetate, 12%; acetyl triethylcitrate, 9%; and dinitrophenoxyethanol, 9%binder components based onpropellant.

A sealer coat was applied to the cylindrical surface by dipping thegrain into liquid acetyl triethyl citrate at room temperature.Commercial powdery cellulose acetate, lacquer grade, was dusted over thewetted surface. The grain was then placed in an oven and held at 110 C.for one hour. The propellant was covered with a hard, tough, extremelythin coat after the baking treatment. This coated grain was thenpositioned in a steel shell and a polyurethane restrictor appliedbetween the sealer coat and the vessel wall by casting technique. Thepolyurethane restrictor material and the casting pro cedure were thesame as described in US. Patent No. 3,012,507 at column 11, series A.

The gas generators were then tested for quality of the restrictor bybeing fired after the standard temperature cycling test. In this test,the gas generator was rapidly cooled from room temperature to -75 F.(-59.5 C.) and held at that temperature for 16 hours; the cold generatorwas then rapidly raised to 170 F. (79.4 C.) and held for 16 hours atthis temperature; the vessel was then cooled to room temperature andfired. A time-pressure tracing was obtained during the firing and theshape of the curve illustrated the effectiveness of the restrictorcoating. Six separate tests on gas generators prepared as describedabove showed no restrictor failures.

Tests were also made on gas generators wherein the sealer coat wasderived from a solution of cellulose acetate in acetone painted on thesurface of the propellant grain. In every instance, the time-pressurefiring trace showed pressure surges which are conclusive proof that thesealer coat had failed at one or more sectors of the grain surface.Attempts to prepare satisfactory gas generators by the use of sealercoats laid down from cellulose acetate bodied citrate plasticizer werealso failures. These tests demonstrate that the article of the inventionwherein a cellulose acetate sealer coat is applied by the proceduredescribed herein solved a very ticklish restrictor coating problem andsuch an article is now .in successful use on a routine production basis.

The drawing shows a side View, and a cross-sectional view taken at 22,of a cylindrical ammonium nitrate based propellant grain it to which hasbeen applied as described herein two sealer coats 12a and 12b.

. Thus having described the invention, what is claimed 1s:

1. An article consisting essentially of an ammonium nitrate body havinga portion of the surface of said body covered with a cellulose acetatecoat, said coat having been obtained by (a) applying to the surface tobe covered a layer of a liquid capable of plasticizing celluloseacetate, (b) applying powdered cellulose acetate to said liquid coveredsurface in an amount sufficient to form a seal coat thereon and (0)baking the acetate covered surface at a temperature of about 120 C. fora time suflicient to form a seal coat of cellulose acetate on saidsurface.

2. The article of claim 1 wherein said body consists essentially ofammonium nitrate, an oXidizable organic binder therefor and an ammoniumnitrate combustion catalyst.

3. A solid propellant consisting essentially of (l) a shaped bodyportion formed of ammonium nitrate as the major component and anoxidizable organic binder therefor, the surface of said body beingcharacterized by the presence of exposed ammonium nitrate which has notbeen covered by said binder and (II) a sealer coat adhered to andadapted to seal that part of the surface of said body where directburning is to be prevented, where said sealer coat has been obtained by(a) applying to the surface to be covered a layer of a liquid capable ofplasticizing cellulose acetate, (b) applying powdered cellulose acetateto said liquid covered surface in an amount sufiicient to form a sealcoat thereon and (c) baking the acetate covered surface at a temperatureof about 90- C. for a time sufficient to form a seal coat of celluloseacetate on said surface.

4. The propellant of claim 3 where a combustion restrictor coat isadhered to said sealer coat, said sealer coat barring contact betweensaid exposed nitrate and said restrictor coat.

5. The propellant of claim 4 Where said liquid is acetyl triethylcitrate.

6. The propellant of claim 5 wherein said cellulose acetate has acombined acetic acid content of about 5058 weight percent.

No references cited.

3. A SOLID PROPELLANT CONSISTING ESSENTIALL OF (I) A SHAPED BODY PORTIONFORMED OF AMMONIUM NITRATE AS THE MAJOR COMPONENT AND AN OXIDIZABELORGANIC BINDER THEREFOR, THE SURFACE OF SAID BODY BEING CHARACTERIZED BYTHE PRESENCE OF EXPOSED AMMONIUM NITRATE WHICH HAS NOT BEEN COVERED BYSAID BINDER AND (II) A SEALER COAT ADHERED TO AND ADAPTED TO SEAL THATPART OF THE SURFACE OF SAID BODY WHERE DIRECT BURNING IS TO BEPREVENTED, WHERE SAID SEALER COAT HAS BEEN OBTAINED BY (A) APPLYING TOTHE SURFACE TO BE COVERED A LAYER OF LIQUID CAPABLE OF PLASTICIZINGCELLULOSE ACETATE, (B) APPLYING POWDERED CELLULOSE ACETATE TO SAIDLIQUID COVERED SURFACE IN AN AMOUNT SUFFICIENT TO FORM A SEAL COATTHEREON AND (C) BAKING THE ACETATE COVERED SURFACE AT A TEMPERATURE OFABOUT 90*120*C. FOR A TIME SUFFICIENT TO FORM A SEAL COAT OF CELLULOSEACETATE ON SAID SURFACE.